Add trait-incrementer example smart contract

[Robbepop]

New example ink! smart contract extracted from #665.

[codecov-commenter]

Codecov Report

Merging #932 (8872977) into master (41b0a1c) will increase coverage by 2.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
+ Coverage   84.20%   86.21%   +2.00%     
==========================================
  Files         173      156      -17     
  Lines        7939     6353    -1586     
==========================================
- Hits         6685     5477    -1208     
+ Misses       1254      876     -378     

Impacted Files	Coverage Δ
crates/env/src/engine/off_chain/db/chain_spec.rs	79.54% <0.00%> (-20.46%)	⬇️
crates/lang/ir/src/ir/item_impl/mod.rs	73.58% <0.00%> (-17.80%)	⬇️
crates/env/src/engine/off_chain/chain_extension.rs	57.14% <0.00%> (-7.86%)	⬇️
crates/lang/codegen/src/generator/metadata.rs	90.90% <0.00%> (-6.66%)	⬇️
crates/env/src/engine/off_chain/db/events.rs	93.54% <0.00%> (-6.46%)	⬇️
crates/lang/ir/src/ir/item_impl/impl_item.rs	51.35% <0.00%> (-6.10%)	⬇️
crates/env/src/engine/off_chain/db/accounts.rs	81.15% <0.00%> (-5.43%)	⬇️
crates/lang/ir/src/ir/idents_lint.rs	68.42% <0.00%> (-3.01%)	⬇️
crates/env/src/engine/off_chain/mod.rs	86.95% <0.00%> (-2.66%)	⬇️
crates/lang/ir/src/ir/config.rs	88.88% <0.00%> (-2.61%)	⬇️
... and 60 more

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[HCastano]

Small comments from my end, but looks good 👍

[HCastano]

My nitpick here was that it's up to the trait implementer to choose the amount inc increases by 😉

[Robbepop]

I do not agree because this would violate the invariant stated by the docs of the trait's message.

[Robbepop]

Just because our concrete incrementer does _also_support inc_by does not mean this invariant must not be uphold for the trait implementation in particular. You could say the demonstrated concrete incrementer smart contract provides a richer API than what the ink! trait mandates but that's always fine, also in typical Rust contexts.